In any transmission wherein a connection is first established before the transmission takes place, bandwidth is reserved along the path used by the connection and error checking is taken into consideration along the path. The protocols using such an approach use a call-connect packet to initiate a session and a connect-confirm response packet to complete the call sequence.
The requirements of connection-oriented systems are that the route is determined at call set up time by allocating a virtual circuit between the two endpoints. At that time, all necessary resources on the virtual circuit are reserved and logical channels are allocated. It is only when the connection is cleared that the resources and logical channels are released.
For example, with Asynchronous Transfer Mode (ATM) networks, a call set up process is established using virtual paths/virtual circuits (VP/VC). All ATM communications are set up by using a controlled method which identifies the rights needed to establish each connection. Generally, connections are not established by end users but by the network devices or nodes. However, there is a trend today to use packet-switched telecommunications directly at IP (Internet Protocol) level which allows the end-users to establish directly the connections.
A connectionless transmission used for example at IP level, is a form of packet-transmission not requiring communications between the end devices before the transmission of data and therefore, is well-adapted to transmit short messages composed of a limited number of packets. Therefore, it is a data transfer without the use of virtual circuit. In simple bus or ring networks, there is no problem implementing connectionless systems because the path-choice is limited. However in meshed and complex networks, the problems are significantly different. Each router must have a large amount of intelligence to process the packet header, and the network requires an efficient mechanism to ensure that all routers or nodes have an up-to-date view of the overall topology.
The Resource Reservation Protocol (RSVP) is a network-control protocol that enables IP applications to obtain special Qualities of Service (QoS) for their data flows. It allows establishment of connection-oriented like communications with quality of service. But RSVP is not a routing protocol. Instead, it works in conjunction with routing protocols and installs the equivalent of dynamic access lists along the routes that routing protocols calculate. RSVP can be used by end-users to reserve bandwidth on the path to the destination on all involved routers. The limitation is that if the bandwidth is already used, there is no way to add more reserved communications. No control on the rights of the end-user to ask for bandwidth is provided.
Another difficulty with a current reservation protocol such as RSVP is that there is not enough scalability since each request has to be handled by each node in the path used by the connection. This problem is very important today when the path is established in a complex networking environment wherein the nodes being used in the path are in several networks which belong to different service and network providers.